I. Field of the Invention
The present invention relates generally to an opposed piston internal combustion four-cycle engine, and more particularly to the piston construction of such an engine.
II. Description of the Prior Art
There have been a number of previously known opposed piston internal combustion engines, mainly for two-cycle operation. In an opposed piston engine, a pair of piston members are slidably disposed in a facing relationship within a single engine cylinder, so that a combustion chamber is defined between the heads of the piston members. The piston members reciprocate away from and towards each other in synchronism, and in doing so provide the driving output of the engine. The heads of the piston members in such prior opposed piston engines have been substantially flat.
Prior to my U.S. Pat. Nos. 4,198,946 and 4,244,338, opposed piston engines were subject to several drawbacks which did not allow them to enjoy widespread commercial success. For example, the prior opposed piston four-cycle engines presented particular difficulties with the valving of both inducting the fuel-air mixture into the combustion chamber, and exhausting the combustion products from the engine cylinder. Previous attempts to adapt poppet valves proved largely unsuccessful, since the poppet valves could not be positioned above the piston head, as in more conventional internal combustion engines. These problems were particularly acute with opposed piston diesel engines. Prior to the inventions disclosed in these patents, the clearance space between the piston heads at maximum compression was too small to enable the use of poppet valves, so that opposed piston diesel engines were previously thought not possible. The inventions disclosed in those patents overcame these drawbacks and provided an opposed piston diesel engine including synchronized opposed pistons, and rotary valves positioned within the intake and exhaust passageways in communication with the combustion chamber, selectively opening and closing the respective passageways in synchrony with the piston cycle.
While my prior patents successfully provided a generally satisfactory opposed piston diesel engine, that engine still encountered some of the drawbacks of conventional diesel engines. A glow plug or other ignition means was still required to successfully initiate ignition of the fuel-air mixture when the engine was cold. Also, for optimal horsepower output from the engine, it was necessary to balance the compression ratio of the engine against the diameters of the pistons, since proportionately more heat is required to induce combustion as the diameter of the pistons increases. Prior attempts to cure this problem in conventional diesel engines have included means for swirling the fuel-air mixture chamber, but these have increased the complexity of the engine, and the associated costs of manufacturing the pistons. Particularly, when the engine is cold, prior glow plugs have been required to operate for a disadvantageously long time, prior to sustained diesel ignition.